My apologies. I should have been more precise.
I mean the corners of those black frames that sorround the forum selection for instance.
The advertisement window on this forum has a rounded frame too.
I hope it's more clear now. Unfortunately I'm not yet allowed to post pictures/urls.

I had the problem with no round corners as well, and I downloaded the corners from the phpBB board and got all of them back except the ones at the very top of the header. My board is at forum dot playselectbaseball dot com. (Sorry, I can't post URLs.) Can someone please help?